The invention concerns a flat membrane of polyamide, copolyamide or of a mixture of polyamides, suitable for ultrafiltration and composed of an ultrafiltration skin and a protective layer.
Hollow fiber membranes of polyamide are known from U.S. Pat. No. 4,340,481, which are cast, for example, from a casting solution containing 15 to 25% by weight polyamide dissolved in formic acid. Core-forming substances, as those explained and described in U.S. Pat. No. 4,340,481 (Col. 6, lines 29-54) acting as pore-formers, for example, metal salt, are added to this solution. After the casting, the cast solution is led into a coagulation bath. After the finish of coagulation, the core-forming substances are washed out whereby the desired pores of the membrane are produced. This known membrane has the form of a hollow fiber and displays moreover, a web of fine filter canals distributed uniformly over the cross-section of the membrane. This known membrane does not display an ultrafiltration skin. Moreover, these membranes display very differing pore sizes. From the pore distribution curve according to FIG. 7 of U.S. Pat. No. 4,340,481 is to be recognized, namely, that compared to the average pore size, pores are contained in the membrane with substantially greater dimensions. As a result, upon employment of this membrane, no sharp separation is obtained during the ultrafiltration.
A microporous membrane is described in U.S. Pat. No. 3,876,738, which is manufactured from a solution which contains polyamide dissolved in formic acid. A relatively higher portion of a non-solvent for polyamide must still be added to this solution. The membrane displays no ultrafiltration skin, and the pore size is supposed to be uniform across the cross-section of the membrane.
A flat, anisotropic membrane without ultrafiltration skin, suitable for ultrafiltration, is known from DE-OS No. 28 45 797. A casting solution, which can be composed of polyamide dissolved in formic acid, must be purposely decomposed before the casting. Upon employment of the polyamide dissolved in formic acid, is provided for the membrane prepared therefrom a low exclusion limit for molecular weight between 300 and 2000. In order to make the membrane suitable for larger molecular weights, another solvent must be employed. According to DE-OS No. 28 45 797, formic acid is thus, not suitable as a solvent for the preparation of membranes with higher exclusion limits.